Ordering kesterite improves solar cells: A low temperature post-deposition annealing study

• We investigated CZTSe solar cells with different ordering degree of kesterite.
• Voc followed the (dis)order induced band gap changes leading to a constant Voc deficit.
• The large Voc deficit is not affected by the kesterite ordering degree.
• Devices based on ordered CZTSe showed a significant improvement in efficiency.

Low temperature (100 and 250 °C) post-deposition annealings were applied to Cu2ZnSnSe4 (CZTSe) absorbers to modify the ordering degree of the kesterite. Spectrophotometry and photoluminescence revealed a blue or red shift of the band gap and the luminescence peak after ordering or disordering post-treatments, respectively. For most solar cells, the open circuit voltage was found to be limited by bulk recombination and to follow band gap variations, leading to a constant open circuit voltage deficit. For the range of ordering degree investigated, the large open circuit voltage deficit, typically encountered in kesterite solar cells, could not be attributed to disorder. Capacitance–voltage measurements demonstrated that the net carrier concentration of ordered CZTSe is reduced by one order of magnitude compared to disordered CZTSe. Besides its beneficial effects on the open circuit voltage, the ordering post-treatment was also found to increase the collection of photo-generated carriers, resulting in a high current density and a significant improvement in device efficiency.